'\" et
.TH SEMOP "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
semop
\(em XSI semaphore operations
.SH SYNOPSIS
.LP
.nf
#include <sys/sem.h>
.P
int semop(int \fIsemid\fP, struct sembuf *\fIsops\fP, size_t \fInsops\fP);
.fi
.SH DESCRIPTION
The
\fIsemop\fR()
function operates on XSI semaphores (see the Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 4.17" ", " "Semaphore").
It is unspecified whether this function interoperates with the
realtime interprocess communication facilities defined in
.IR "Section 2.8" ", " "Realtime".
.P
The
\fIsemop\fR()
function shall perform atomically a user-defined array of semaphore
operations in array order on the set of semaphores associated with the
semaphore identifier specified by the argument
.IR semid .
.P
The argument
.IR sops
is a pointer to a user-defined array of semaphore operation
structures. The implementation shall not modify elements of this array
unless the application uses implementation-defined extensions.
.P
The argument
.IR nsops
is the number of such structures in the array.
.P
Each structure,
.BR sembuf ,
includes the following members:
.TS
center box tab(!);
cB | cB | cB
lw(1.25i)B | lw(1.25i)I | lw(2.5i).
Member Type!Member Name!Description
_
unsigned short!sem_num!Semaphore number.
short!sem_op!Semaphore operation.
short!sem_flg!Operation flags.
.TE
.P
Each semaphore operation specified by
.IR sem_op
is performed on the corresponding semaphore specified by
.IR semid
and
.IR sem_num .
.P
The variable
.IR sem_op
specifies one of three semaphore operations:
.IP " 1." 4
If
.IR sem_op
is a negative integer and the calling process has alter permission, one
of the following shall occur:
.RS 4 
.IP " *" 4
If
.IR semval (see
.IR <sys/sem.h> )
is greater than or equal to the absolute value of
.IR sem_op ,
the absolute value of
.IR sem_op
is subtracted from
.IR semval .
Also, if (\fIsem_flg\fP &SEM_UNDO) is non-zero, the absolute value of
.IR sem_op
shall be added to the
.IR semadj
value of the calling process for the specified semaphore.
.IP " *" 4
If
.IR semval
is less than the absolute value of
.IR sem_op
and (\fIsem_flg\fP &IPC_NOWAIT) is non-zero,
\fIsemop\fR()
shall return immediately.
.IP " *" 4
If
.IR semval
is less than the absolute value of
.IR sem_op
and (\fIsem_flg\fP &IPC_NOWAIT) is 0,
\fIsemop\fR()
shall increment the
.IR semncnt
associated with the specified semaphore and suspend execution of the
calling thread until one of the following conditions occurs:
.RS 4 
.IP -- 4
The value of
.IR semval
becomes greater than or equal to the absolute value of
.IR sem_op .
When this occurs, the value of
.IR semncnt
associated with the specified semaphore shall be decremented, the
absolute value of
.IR sem_op
shall be subtracted from
.IR semval
and, if (\fIsem_flg\fP &SEM_UNDO) is non-zero, the absolute value of
.IR sem_op
shall be added to the
.IR semadj
value of the calling process for the specified semaphore.
.IP -- 4
The
.IR semid
for which the calling thread is awaiting action is removed from the
system. When this occurs,
.IR errno
shall be set to
.BR [EIDRM] 
and \-1 shall be returned.
.IP -- 4
The calling thread receives a signal that is to be caught. When this
occurs, the value of
.IR semncnt
associated with the specified semaphore shall be decremented, and the
calling thread shall resume execution in the manner prescribed in
.IR "\fIsigaction\fR\^(\|)".
.RE
.RE
.IP " 2." 4
If
.IR sem_op
is a positive integer and the calling process has alter permission, the
value of
.IR sem_op
shall be added to
.IR semval
and, if (\fIsem_flg\fP &SEM_UNDO) is non-zero, the value of
.IR sem_op
shall be subtracted from the
.IR semadj
value of the calling process for the specified semaphore.
.IP " 3." 4
If
.IR sem_op
is 0 and the calling process has read permission, one of the following
shall occur:
.RS 4 
.IP " *" 4
If
.IR semval
is 0,
\fIsemop\fR()
shall return immediately.
.IP " *" 4
If
.IR semval
is non-zero and (\fIsem_flg\fP &IPC_NOWAIT) is non-zero,
\fIsemop\fR()
shall return immediately.
.IP " *" 4
If
.IR semval
is non-zero and (\fIsem_flg\fP &IPC_NOWAIT) is 0,
\fIsemop\fR()
shall increment the
.IR semzcnt
associated with the specified semaphore and suspend execution of the
calling thread until one of the following occurs:
.RS 4 
.IP -- 4
The value of
.IR semval
becomes 0, at which time the value of
.IR semzcnt
associated with the specified semaphore shall be decremented.
.IP -- 4
The
.IR semid
for which the calling thread is awaiting action is removed from the
system. When this occurs,
.IR errno
shall be set to
.BR [EIDRM] 
and \-1 shall be returned.
.IP -- 4
The calling thread receives a signal that is to be caught. When this
occurs, the value of
.IR semzcnt
associated with the specified semaphore shall be decremented, and the
calling thread shall resume execution in the manner prescribed in
.IR "\fIsigaction\fR\^(\|)".
.RE
.RE
.P
Upon successful completion, the value of
.IR sempid
for each semaphore specified in the array pointed to by
.IR sops
shall be set to the process ID of the calling process. Also, the
.IR sem_otime
timestamp shall be set to the current time, as described in
.IR "Section 2.7.1" ", " "IPC General Description".
.SH "RETURN VALUE"
Upon successful completion,
\fIsemop\fR()
shall return 0; otherwise, it shall return \-1 and set
.IR errno
to indicate the error.
.SH ERRORS
The
\fIsemop\fR()
function shall fail if:
.TP
.BR E2BIG
The value of
.IR nsops
is greater than the system-imposed maximum.
.TP
.BR EACCES
Operation permission is denied to the calling process; see
.IR "Section 2.7" ", " "XSI Interprocess Communication".
.TP
.BR EAGAIN
The operation would result in suspension of the calling process but
(\fIsem_flg\fP &IPC_NOWAIT) is non-zero.
.TP
.BR EFBIG
The value of
.IR sem_num
is greater than or equal to the number of semaphores in the set
associated with
.IR semid .
.TP
.BR EIDRM
The semaphore identifier
.IR semid
is removed from the system.
.TP
.BR EINTR
The
\fIsemop\fR()
function was interrupted by a signal.
.TP
.BR EINVAL
The value of
.IR semid
is not a valid semaphore identifier, or the number of individual
semaphores for which the calling process requests a SEM_UNDO would
exceed the system-imposed limit.
.TP
.BR ENOSPC
The limit on the number of individual processes requesting a SEM_UNDO
would be exceeded.
.TP
.BR ERANGE
An operation would cause a
.IR semval
to overflow the system-imposed limit, or an operation would cause a
.IR semadj
value to overflow the system-imposed limit.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Setting Values in Semaphores"
.P
The following example sets the values of the two semaphores associated
with the
.IR semid
identifier to the values contained in the
.IR sb
array.
.sp
.RS 4
.nf

#include <sys/sem.h>
\&...
int semid;
struct sembuf sb[2];
int nsops = 2;
int result;
.P
/* Code to initialize semid. */
\&...
.P
/* Adjust value of semaphore in the semaphore array semid. */
sb[0].sem_num = 0;
sb[0].sem_op = -1;
sb[0].sem_flg = SEM_UNDO | IPC_NOWAIT;
sb[1].sem_num = 1;
sb[1].sem_op = 1;
sb[1].sem_flg = 0;
.P
result = semop(semid, sb, nsops);
.fi
.P
.RE
.SS "Creating a Semaphore Identifier"
.P
The following example gets a unique semaphore key using the
\fIftok\fR()
function, then gets a semaphore ID associated with that key using the
\fIsemget\fR()
function (the first call also tests to make sure the semaphore exists).
If the semaphore does not exist, the program creates it, as shown by
the second call to
\fIsemget\fR().
In creating the semaphore for the queuing process, the program
attempts to create one semaphore with read/write permission for all. It
also uses the IPC_EXCL flag, which forces
\fIsemget\fR()
to fail if the semaphore already exists.
.P
After creating the semaphore, the program uses calls to
\fIsemctl\fR()
and
\fIsemop\fR()
to initialize it to the values in the
.IR sbuf
array. The number of processes that can execute concurrently without
queuing is initially set to 2. The final call to
\fIsemget\fR()
creates a semaphore identifier that can be used later in the program.
.P
Processes that obtain
.IR semid
without creating it check that
.IR sem_otime
is non-zero, to ensure that the creating process has completed the
\fIsemop\fR()
initialization.
.P
The final call to
\fIsemop\fR()
acquires the semaphore and waits until it is free; the SEM_UNDO option
releases the semaphore when the process exits, waiting until there are
less than two processes running concurrently.
.br
.sp
.RS 4
.nf

#include <stdio.h>
#include <sys/sem.h>
#include <sys/stat.h>
#include <errno.h>
#include <stdlib.h>
\&...
key_t semkey;
int semid;
struct sembuf sbuf;
union semun {
    int val;
    struct semid_ds *buf;
    unsigned short *array;
} arg;
struct semid_ds ds;
\&...
/* Get unique key for semaphore. */
if ((semkey = ftok("/tmp", \(aqa\(aq)) == (key_t) -1) {
    perror("IPC error: ftok"); exit(1);
}
.P
/* Get semaphore ID associated with this key. */
if ((semid = semget(semkey, 0, 0)) == -1) {
.P
    /* Semaphore does not exist - Create. */
    if ((semid = semget(semkey, 1, IPC_CREAT | IPC_EXCL | S_IRUSR |
        S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH)) != -1)
    {
        /* Initialize the semaphore. */
        arg.val = 0;
        sbuf.sem_num = 0;
        sbuf.sem_op = 2;  /* This is the number of runs without queuing. */
        sbuf.sem_flg = 0;
        if (semctl(semid, 0, SETVAL, arg) == -1
            || semop(semid, &sbuf, 1) == -1) {
            perror("IPC error: semop"); exit(1);
        }
    }
    else if (errno == EEXIST) {
        if ((semid = semget(semkey, 0, 0)) == -1) {
            perror("IPC error 1: semget"); exit(1);
        }
        goto check_init;
    }
    else {
        perror("IPC error 2: semget"); exit(1);
    }
}
else
{
    /* Check that semid has completed initialization. */
    /* An application can use a retry loop at this point rather than
       exiting. */
    check_init:
    arg.buf = &ds;
    if (semctl(semid, 0, IPC_STAT, arg) < 0) {
        perror("IPC error 3: semctl"); exit(1);
    }
    if (ds.sem_otime == 0) {
        perror("IPC error 4: semctl"); exit(1);
    }
}
\&...
sbuf.sem_num = 0;
sbuf.sem_op = -1;
sbuf.sem_flg = SEM_UNDO;
if (semop(semid, &sbuf, 1) == -1) {
    perror("IPC Error: semop"); exit(1);
}
.fi
.P
.RE
.SH "APPLICATION USAGE"
The POSIX Realtime Extension defines alternative interfaces for interprocess
communication. Application developers who need to use IPC should
design their applications so that modules using the IPC routines
described in
.IR "Section 2.7" ", " "XSI Interprocess Communication"
can be easily modified to use the alternative interfaces.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.7" ", " "XSI Interprocess Communication",
.IR "Section 2.8" ", " "Realtime",
.IR "\fIexec\fR\^",
.IR "\fIexit\fR\^(\|)",
.IR "\fIfork\fR\^(\|)",
.IR "\fIsemctl\fR\^(\|)",
.IR "\fIsemget\fR\^(\|)",
.IR "\fIsem_close\fR\^(\|)",
.IR "\fIsem_destroy\fR\^(\|)",
.IR "\fIsem_getvalue\fR\^(\|)",
.IR "\fIsem_init\fR\^(\|)",
.IR "\fIsem_open\fR\^(\|)",
.IR "\fIsem_post\fR\^(\|)",
.IR "\fIsem_trywait\fR\^(\|)",
.IR "\fIsem_unlink\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "Section 4.17" ", " "Semaphore",
.IR "\fB<sys_ipc.h>\fP",
.IR "\fB<sys_sem.h>\fP",
.IR "\fB<sys_types.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
